Influence of dynamic inflow on the helicopter vertical response

A study has been conducted to investigate the effects of dynamic inflow on rotor-blade flapping and vertical motion of the helicopter in hover. Linearized versions of two dynamic inflow models, one developed by Carpenter and Fridovich and the other by Pitt and Peters, were incorporated in simplified rotor-body models and were compared for variations in thrust coefficient and the blade Lock number. In addition, a comparison was made between the results of the linear analysis, and the transient and frequency responses measured in flight on the CH-47B variable-stability helicopter. Results indicate that the correlations are good, considering the simplified model used. The linear analysis also shows that dynamic inflow plays a key role in destabilizing the flapping mode. The destabilized flapping mode, along with the inflow mode that the dynamic inflow introduces, results in a large initial overshoot in the vertical acceleration response to an abrupt input in the collective pitch. This overshoot becomes more pronounced as either the thrust coefficient or the blade Lock number is reduced. Compared with Carpenter's inflow model, Pitt's model tends to produce more oscillatory responses because of the less stable flapping mode predicted by it.

[1]  A. E. Bryson,et al.  The inclusion of rotor dynamics in controller design for helicopters. , 1972 .

[2]  D. C. Dugan,et al.  Frequency-Domain Identification of XV-15 Tilt-Rotor Aircraft Dynamics in Hovering Flight , 1985 .

[3]  Kenneth H. Landis,et al.  Development of ADOCS controllers and control laws. Volume 1: Executive summary , 1985 .

[4]  William S. Hindson,et al.  New Capabilities and Recent Research Programs of the NASA/Army CH-47B Variable-Stability Helicopter , 1986 .

[5]  R. H. Miller,et al.  A Method for Improving the Inherent Stability and Control Characteristics of Helicopters , 1950 .

[6]  Mark B. Tischler,et al.  Frequency-response identification of XV-15 tilt-rotor aircraft dynamics , 1983 .

[7]  Wayne Johnson,et al.  Influence of unsteady aerodynamics on hingeless rotor ground resonance , 1982 .

[8]  William S. Hindson,et al.  Flight investigation of a multivariable model-following control system for rotorcraft , 1986 .

[9]  R. T. N. Chen,et al.  A simplified rotor system mathematical model for piloted flight dynamics simulation , 1979 .

[10]  Edward Seckel,et al.  Stability and Control of Airplanes and Helicopters , 1964 .

[11]  Duane T. McRuer,et al.  A perspective on superaugmented flight control - Advantages and problems , 1986 .

[12]  Robert A. Ormiston.  Technical Notes: Application of Simplified Inflow Models to Rotorcraft Dynamic Analysis , 1976 .

[13]  R. T. N. Chen,et al.  Effects of primary rotor parameters on flapping dynamics , 1980 .

[14]  David A. Peters,et al.  Theoretical prediction of dynamic inflow derivatives , 1980 .

[15]  W. S. Hindson,et al.  Influence of high-order dynamics on helicopter flight-control system bandwidth , 1986 .

[16]  Paul J Carpenter,et al.  Effect of a Rapid Blade-Pitch Increase on the Thrust and Induced-Velocity Response of a Full-Scale Helicopter Rotor , 1953 .

[17]  David A. Peters,et al.  A review of dynamic inflow and its effect on experimental correlations , 1985 .

[18]  Peretz P. Friedmann,et al.  Influence of various unsteady aerodynamic models on the aeromechanical stability of a helicopter in ground resonance , 1984 .